As new generations of cellular phones and other wireless communication devices become smaller and embedded with increased applications, new antenna designs are required to address inherent limitations of these devices and to enable new capabilities. With conventional antenna structures, a certain physical volume is required to produce a resonant antenna structure at a particular frequency and with a particular bandwidth. However, effective implementation of such antennas is often confronted with size constraints due to a limited available space in the device.
Antenna efficiency is one of the important parameters that determine the performance of the device. In particular, radiation efficiency is a metric describing how effectively the radiation occurs, and is expressed as the ratio of the radiated power to the input power of the antenna. A more efficient antenna will radiate a higher proportion of the energy fed to it. Likewise, due to the inherent reciprocity of antennas, a more efficient antenna will convert more of a received energy into electrical energy. Therefore, antennas having both good efficiency and compact size are often desired for a wide variety of applications.
Conventional loop antennas are typically current fed devices, which generate primarily a magnetic (H) field. As such, they are not typically suitable as transmitters. This is especially true of small loop antennas (i.e. those smaller than, or having a diameter less than, one wavelength). The amount of radiation energy received by a loop antenna is, in part, determined by its area. Typically, each time the area of the loop is halved, the amount of energy which may be received is reduced by approximately 3 dB. Thus, the size-efficiency tradeoff is one of the major considerations for loop antenna designs.
Voltage fed antennas, such as dipoles, radiate both electric (E) and H fields and can be used in both transmit and receive modes. Compound antennas are those in which both the transverse magnetic (TM) and transverse electric (TE) modes are excited, resulting in performance benefits such as wide bandwidth (lower Q), large radiation intensity/power/gain, and good efficiency. There are a number of examples of two dimensional, non-compound antennas, which generally include printed strips of metal on a circuit board. Most of these antennas are voltage fed. An example of one such antenna is the planar inverted F antenna (PIFA). A large number of antenna designs utilize quarter wavelength (or some multiple of a quarter wavelength), voltage fed, dipole antennas.
Use of MIMO (multiple input multiple output) technologies is increasing in today's wireless communication devices to provide enhanced data communication rates while minimizing error rates. A MIMO system is designed to mitigate interference from multipath environments by using several transmit (Tx) antennas at the same time to transmit different signals, which are not identical but are different variants of the same message, and several receive (Rx) antennas at the same time to receive the different signals. A MIMO system can generally offer significant increases in data throughput without additional bandwidth or increased transmit power by spreading the same total transmit power over the antennas so as to achieve an array gain. MIMO protocols constitute a part of wireless communication standards such as IEEE 802.11n (Wi-Fi), 4G, Long Term Evolution (LTE), WiMAX and HSPA+. However, in a configuration with multiple antennas, size constraints tend to become severe, and interference effects caused by electromagnetic coupling among the antennas may significantly deteriorate transmission and reception qualities. At the same time, efficiency may deteriorate in many instances where multiple paths are energized and power consumption increases.
Such deterioration in transmission and reception quality is particularly problematic for WLAN and WAN applications as coverage, effective throughput delivered at any point, and the level of interference, and capacity are particularly important parameters for effective wideband connectivity. The performance expectations for WLAN and WAN with new applications such as casual video streaming are very high and antennas used in such systems must be able to address these performance issues to improve performance.
There are many opportunities to improve the performance as experienced by the end-user whether on a WAN with his smart phone or on private or public WLAN network with his PC or handheld device. Specifically, WLAN has room for improvement in residential deployment in a residential scenario where each dwelling has one or more Wi-Fi access point(s) or router(s). The coverage and throughput at range has improved drastically over the years and, on average, the majority of users get good coverage and throughout data in their home. However, an increasing problem is the scarcity of available bandwidth and the related interference problems increasing each year. The first ISM band for a WLAN use was 900 MHz in the USA. With the abundance of devices such as cameras and cordless phones and various protocols and different modulation schemes, the 900 MHz band became rapidly overcrowded. The 2.4 GHz band was then allocated and used for IEEE 802.11b and 802.11g devices on 11 channels but only 3 non-overlapping channels.
New cordless phones (e.g: Digital Enhanced Cordless Telecommunications DECT 6.0) were also introduced in the 2.4 GHz band and the current status is that, on average, in any metropolitan area or suburb, each WLAN node sees 5 or 10 routers in the vicinity. For routers it means competition for bandwidth since they see each-other as rogue applications. Algorithms for decreasing transmit power and back off (delay) transmission exist, but on the practical side they are not applied too often in Wi-Fi applications. Whereas for LTE, WCDMA there is a constant power adjustment between the mobile handset user equipment (UE) and the evolved node B (enode B), base transceiver station in GSM. On average, the 2.4 GHz band is also overcrowded. The relatively new use of the 5 GHz band provides much more non-overlapping channels but the continuous hunger for more bandwidth exacerbated by the need for more video at a higher definition suggests a need for more bandwidth resources such as 802.11ac with the provision of a 160 MHz bandwidth.
The performance problems with WLAN and WAN systems generally have been addressed by the development of new generation 802.11b/g/a/n wireless cards and modules. However, nearby wireless LANs conflict with each other, due largely to interference cancellation of overlapping channels. This conflict reduces bandwidth as the client gets farther from a router, and closer to competing routers in neighboring WLANs. Also, weak throughput data or underperformance for video applications due to interference remains a problem and provides a major opportunity for performance improvement. Moreover, the current 802.11 systems must be compared with new competing wireless standards such as an LTE-Advanced release 10 that promise up to 3 Gbps of data in download (DL) and up to 1.5 Gbps in upload (UL) for cellular devices, tablets and PC through bridging modes. A major difference is the synchronization aspect of cellular networks such as LTE versus Wi-Fi in residential environments. Interference problems should not exist or be much lower with LTE versus Wi-Fi; however, less than perfect capacity and low throughput due to multipath fading still exists even for a synchronized WAN.
The present applicant has developed logarithmic detector amplifiers (LDAs) that address the above issues by enabling a receiver to receive lower level systems more reliably in the presence of noise. Also, LDAs permit the transmit power of the transmitter to be proportionately decreased to provide lower levels of interference for neighboring devices. An exemplary LDA is described in U.S. Pat. No. 7,911,235, the contents of which are incorporated herein by reference. As explained therein, an LDA includes an amplifier (e.g., a discrete transistor or an operational amplifier), means for setting a frequency of operation of the detector (e.g., a tuned L-C or R-C tuned feedback circuit or phase-locked loop), and a controller. An input signal to the amplifier causes an oscillation in the amplifier, and the controller senses a threshold indicative of oscillation and in response to detecting oscillation interrupts the oscillation of the amplifier such that the frequency of the interruption is proportional to a logarithm of the power of the input signal.
During operation, electrical noise at the input of the amplifier sets up oscillations in the circuit at the frequency determined by the L-C tuned feedback circuit. Noise outside the bandwidth of the tuned circuit has minimal effect on the operation of the circuit. On the other hand, incoming signals lying within the bandwidth of the L-C tuned feedback circuit cause the oscillator to oscillate more rapidly than if random noise alone were exciting the circuit. In the event that there is a wanted signal amongst received noise, the relative level of the input signal at the set frequency of operation is higher and a threshold will be reached sooner than would be the case for random noise alone. The higher the level of the wanted signal, the sooner the threshold is reached and the interruption of the oscillation will be more frequent. Accordingly, wanted signals having the desired frequency cause oscillations to occur more quickly than will random noise alone. Such LDAs have been shown to be quite effective in canceling interference.
It is desired to address the afore-mentioned problems in the art by providing greater spatial diversity. In traditional mobile cellular network systems, the base station has no information on the position of the mobile units within the cell and radiates the signal in all directions within the cell in order to provide radio coverage. This results in wasting power on transmissions when there are no mobile units to reach, in addition to causing interference for adjacent cells using the same frequency, so called co-channel cells. Likewise, in reception, the antenna receives signals coming from all directions including noise and interference signals. By using smart antenna technology and differing spatial locations of mobile units within the cell, space-division multiple access techniques offer attractive performance enhancements. The radiation pattern of the base station, both in transmission and reception, is adapted to each user to obtain highest gain in the direction of that user. This is often done using phased array techniques.
In view of the increased spatial diversity using such antennas and the significant improvements in interference cancellation when using LDAs, it is desired in accordance with the invention to explore the use of LDAs in new applications in wireless and wired communications. In particular, it is desired to synchronize LDAs, multiple antennas, active antennas, and multiple active antennas and receivers to reduce or eliminate interference, thereby providing greater range and bandwidth between wireless routers and their clients. The invention addresses these and other needs in the art.